The HE400i is definitely more transparent and neutral. I think they're nearly equally fun. The speed, planar bass and transparency on the HE400i can be addicting. The HE400i is a better technical headphone while managing to also be very musical. The THX00 sold out for pure enjoyment. It's a joy without really fussing over technical issues, without being overtly bad on technical aspects either. People who want clincality at all costs won't particularly enjoy the THX00, but they probably didn't buy it in the first place either. People who are looking for a great bass headphone, that's a lot of fun, without MAJORLY compromising in other areas will love it.
HE400i (1-10 scale)
Transparency 9/10
Speed 8/10
Bass quality 9/10
bass impact 7/10
soundstage 5/10
imaging accuracy 10/10
midrange quality 7/10
treble quality 7/10
smoothness 8/10
linearity 7/10
fun factor 9/10
comfort 10/10
looks 6/10
Total: 101/130
THX00
transparency 6/10
speed 6/10
bass quality 7/10
bass impact 10/10
soundstage 5/10
imaging accuracy 6/10
midrange quality 6/10
treble quality 6/10
smoothness 9/10
linearity 5/10
fun factor 10/10
comfort 9/10
looks 8/10
Total: 93/130
edit: keep in mind I'm only at 3 hours of burn in at this point (which have all ben "on head" burn in)